The exchange of digital messages between two intercommunicating stations may take place either simultaneously over parallel channels or alternately via a single transmission link. The latter technique, available for short distances usually limited to several kilometers or miles, has become known as the "ping-pong" mode and involves an alternation of incoming and outgoing data words within a predetermined time slot which in a telephone system may have a duration of 125 .mu.s. Since a station receiving an incoming data word can transmit an outgoing data word only after the last incoming bit has arrived, the length of such a data word must be well below half a time slot in order to allow for the necessary transit time between the two stations. Thus, the bits of each data word must be rather closely spaced and must have well-defined time positions for proper decoding at the receiving end.
The requisite synchronization of the two terminals can be achieved most expeditiously with the aid of phase-lock circuits controlled by pulses extracted from the incoming message signals. Such circuits have the advantage of being realizable with relatively inexpensive components that can be readily integrated. The extraction of synchronizing pulses from the incoming signals, however, is more problematical; quartz filters suitable for this purpose, for instance, are rather costly.